This invention relates generally to means for feeding and positioning articles undergoing processing relative to the means carrying out the processing. More particularly, the invention relates to a system for feeding and positioning articles which is designed to carry out with high precision positioning of articles such as lead frames in the fabrication and assembly of semiconductor devices.
In general, the process of assembling a semiconductor device comprises the steps of mounting a semiconductor chip on a lead frame, carrying out wire bonding with respect to this semiconductor chip thus mounted, and setting this semiconductor chip thus wire bonded in a metal mold. In carrying out these steps, a lead frame is fed along guide rails, and, at each work position, it is required that the lead frame be positioned and stopped accurately and positively.
In order to feed and position the lead frame, it has been the practice to form pilot holes at constant spacing intervals along a lateral side part of the lead frame in the longitudinal direction thereof and to cause a pilot pin to engage with each pilot hole thereby to achieve positioning.
In this known technique, however, pilot pins respectively suitable for a variety of lead frames of different frame widths and feed pitches become necessary, whereby the pilot pin must be changed for every frame, and assembly efficiency drops.
For this reason, we have proposed a system (as disclosed in Japanese Patent Application Laid-Open Publication No. 81634/1986) in which: a lead frame is moved through only a set distance; the actual lead frame feed quantity is detected at every completion of the movement; the (error) difference between this detected quantity and a specified lead frame feed quantity is measured by means such as an ITV camera; and the difference thus measured is used as feedback to correct the lead frame feed quantity of the succeeding feeding, whereby the necessity of using pilot pins is obviated.
In another prior-art technique (as disclosed in Japanese Patent Application Laid-Open Publication No. 80231/1985), positioning of a lead frame is accomplished by using an image sensor. More specifically, the positional difference or offset between the detection position and the center position of the island of the lead frame is detected by an image sensor, and the lead frame is moved by a distance corresponding to this positional offset thereby to accomplish correction.
However, in the above described positioning techniques in which pilot pins are unnecessary, it has been heretofore necessary to either use a pattern-recognition ITV camera having the object of pattern recognition of chip mounts, wire bonding, etc., or to use separate ITV cameras. In either case, time for pattern recognition becomes necessary and gives rise to a decrease in cycle time. Moreover, pattern recognition entails a program of relatively high level, the control of which occupies a large proportion thereof, whereby not only does a control system of extensive scale become necessary, but, for adaptability to a large variety of articles, a memory capacity for storing corresponding patterns becomes necessary. Furthermore, in pattern recognition, a pattern of a configuration which is appropriate for the lead frame is required, and its size must match the image size of the ITV camera.
In the above described example of the prior art, an X-Y table is caused to move, and an appropriate shape in the vicinity of the inner lead is made the objective. However, not only does a movement time become necessary, but the inner lead is readily deformed, whereby there arises the problem of uncertainty of reliability of this as a datum reference. While a pilot hole is suitable as a datum reference, it is accompanied by a number of problems such as the great length of the movement distance and the addition of the movement time. Furthermore, the size of a pilot hole is of the order of 1 to 2 mm and does not match the ITV camera in some cases, and, even if it does match, memory means for a great variety of patterns becomes necessary.
The use of separate ITV cameras for respectively exclusive use may appear to be feasible, but it has been accompanied in all instances by the problem of high costs and the problem of excessive installation space due to the positioning above of the head of the chip mounting mechanism and of the head of the wire bonding mechanism and other parts and to the positioning below of a wafer ring, heater, etc.